Nuclear physics in type I X-ray bursts

Abstract

X-ray bursts (XRBs), one of the most fascinating astrophysical phenomena, are characterized by sudden 10–50 dramatic increase in luminosity by the X-ray emission of roughly 10–100 s in duration, with peak luminosities of roughly 1038 erg/s. The XRB is a recurrent phenomenon on time scales of hours to days. It is a violent nuclear process, i.e., thermonuclear runaway, occurs in a close binary system which usually consists of a neutron star (or a black hole) and a less-evolved company “donor” star (usually a red giant star). The accreted material burns stably through both the triple-α reaction and the hot, β-limited carbon-nitrogen-oxygen (HCNO) cycles, giving rise to the persistent flux. Once critical temperatures and densities are achieved in the accretion disk, breakout from this region toward higher masses can occur through α-induced reactions. Subsequently, the rapid-proton capture process (so-called rp-process) drives nucleosynthesis toward the proton drip-line. This eventually results in a rapid increase in energy generation (ultimately leading to the XRB) and nucleosynthesis up to A ~100 mass region. As the most frequent thermonuclear phenomena in the universe, the XRBs have been studied in detail in a number of space-borne X-ray satellite observatory missions, including EXOSAT (1983–1986), RXTE (1995–2012), BeppoSAX (1996–2003), Chandra (1999–2019), XMM-Newton (1999–2020) and HETE-2 (2000–2008). In the year of 2017, the Insight Hard X-ray Modulation Telescope (HXMT) had been launched, as the first X-ray astronomy satellite of China. The main scientific objectives of Insight-HXMT are: (1) To scan the Galactic Plane to find new transient sources and to monitor the known variable sources, (2) to observe X-ray binaries to study the dynamics and emission mechanism in strong gravitational or magnetic fields, and (3) to find and study γ-ray bursts with its anti-coincidence CsI detectors. Up to now, more than 90 galactic XRBs have been identified since their initial discovery in 1976. These observations have provided abundant data and opened a new era in X-ray astronomy. The surface of accreting neutron stars is characterized by a wide range of nuclear processes ranging from fragmentation at infall, over thermonuclear burning in X-ray bursts and superbursts, to electron captures and pycnonuclear fusion reactions in the neutron star crust. These processes involve the majority of the nuclei between the proton and the neutron drip line up to a maximum mass number set by the endpoint of the rp-process. In this picture, the rp-process during X-ray bursts (and maybe during stable hydrogen burning as well) plays a central role as it directly determines one of the key observables, the observed X-ray bursts. To investigate the scientific issues in XRBs, such as the observed luminosity curves, the astrophysical environment parameters in the binary system, and the isotropic abundance distributions in the burst ashes, nuclear physicists need precise nuclear-physics inputs, such as nuclear masses (or reaction Q values), half-lives, as well as thermonuclear reaction rates, and β-decay weak rates. Combined with the astrophysics models, one can understand the physics in XRBs more deeply. This paper systematically reviews the nuclear physics in type I X-ray bursts, including the scientific motivation, importance, major research topics, as well as some progress achieved, etc. Thereinto, the relevant nucleosynthesis processes and key nuclear-physics inputs required, are introduced in detail. The status and progress for the XRBs studies are introduced and prospected for the Chinese institutions. In this review, the goals of different research topics are summarized and evaluated, and the promising direction is guided for future investigations.